Amanda Fischer hugs her daughter, Sally Fischer during a memorial service at the Santa Fe Dam Recreation Area for their friend Larry Daniel Kaufman who was killed during a mass shooting at the Inland Regional Center on December 5, 2015 in Irwindale, California. Photo: Joe Raedle/Getty Images/AFP

President Barack Obama will make a rare primetime address to the nation Sunday laying out how he will keep Americans safe and defeat the Islamic State group, days after 14 people were shot dead in California.

Obama declared Saturday that the United States "will not be terrorized," as IS praised the couple behind a mass shooting in San Bernardino as "soldiers" of its self-proclaimed caliphate.

"We are Americans. We will uphold our values -- a free and open society," Obama said in his weekly radio address.

Investigators are combing over evidence and looking into the background of Syed Farook, 28, and his 29-year-old Pakistani wife Tashfeen Malik, the pair who opened fire at a social services center during a holiday party on Wednesday.

The FBI said federal agents raided a property in Riverside, California, but declined to provide the address.

NBC News reported that the FBI had searched the home of Enrique Marquez, a friend of Farook's who originally bought the assault rifles used in the shooting but who is not considered a suspect.

"I heard the windows crashing, the garage door being torn, doors being broken down, you know, they were forcing their way in," neighbor Freddy Escamilla told AFP.

In his address from the Oval Office on Sunday at 8:00 pm (0100 GMT Monday), Obama will try to reassure Americans in the wake of the shooting, which the FBI is investigating as a possible act of terrorism.

The massacre, if proven to be terror-related, would be the deadliest such assault on American soil since the September 11, 2001 attacks.

The last time Obama gave an Oval Office address was in August 2010 to mark the end of US combat operations in Iraq.

He will give an update on the investigation and "will also discuss the broader threat of terrorism, including the nature of the threat, how it has evolved, and how we will defeat it," a White House statement said.

'Destructive ideology'

"He will reiterate his firm conviction that ISIL (IS) will be destroyed and that the United States must draw upon our values -- our unwavering commitment to justice, equality and freedom -- to prevail over terrorist groups that use violence to advance a destructive ideology."

Top security officials have indicated that the pair had been radicalized. But the White House and the FBI say there are no signs they were part of a larger group or terrorist cell.

In a radio broadcast in English, IS praised the couple as "soldiers of the caliphate" and martyrs, but did not specifically say they were members of the extremist group.

The heavily armed couple, who also wounded 21 in their attack, died in a ferocious shootout with police after a huge manhunt.

The rampage was the worst in the United States in three years and revived impassioned debate on gun control in a country where such mass killings have become routine.

The New York Times published a front-page editorial -- the first since 1920 -- calling for an end to "the gun epidemic in America."

Authorities say US-born Farook and Malik, who married last year in Saudi Arabia where she lived, carefully planned their attack.

David Bowdich, the assistant FBI director in charge of the Los Angeles office, said investigators were examining a Facebook posting in which Malik is believed to have pledged allegiance to IS leader Abu Bakr al-Baghdadi, made around the time of the attack.

The family's attorneys said that while the two were devout Muslims, there was no hint they had become radicalized.

Relatives of Farook and his wife have also been at a loss to explain what triggered the killing spree, describing them as a quiet couple who kept to themselves.

They had a six-month-old daughter, whom they left with Farook's mother before the shooting.

"How can he leave his only child, you know? And how could the mother do this?" one of Farook's sisters, Saira Khan, told The New York Times.

Washington has long warned about the threat of homegrown self-radicalized extremists.

"We have moved to an entirely new phase in the global terrorist threat and in our homeland security efforts," Homeland Security Secretary Jeh Johnson told the Times.

Terrorists have "in effect outsourced attempts to attack our homeland. We've seen this not just here but in other places," he added.

"This requires a whole new approach, in my view."

Donald Trump, the frontrunner for the 2016 Republican presidential nomination, gave little doubt as to how he would deal with any terror threat from within.

"I would handle it so tough, you don't want to hear," CNN quoted him as saying.